The present invention relates to integrated circuit fabrication methods and structures, and particularly to integrated circuits with minimum linewidths below one-half micron.
The disclosed innovations are particularly useful for the sub-half-micron regime, and are thus particularly relevant to product generations such as 16M SRAMs and 64M DRAMs.
Practically all technologies to date use a sidewall SiO.sub.2 spacer on the polycide gate for lightly doped drain formation. However, this leads to some problems, since the deposition of oxides in narrow spaces is normally not perfectly conformal, and since loading effects during the spacer etch may cause slower etching of large areas of oxide. Significant overetch of the field oxide regions therefore occurs during the etch step which forms the sidewall oxides. This erosion of the field oxide may be sufficient to permit the implant which forms the heavily doped source/drain regions to partially penetrate the field oxide (and therefore undesirably lower the threshold voltage of the parasitic thick-field transistors, and therefore degrade the effectiveness of isolation). Also, because the spacer is made of oxide, a design rule for minimum contact-to-gate spacing must be maintained so that the contact does not short to the gate due to alignment tolerances. This minimum contact-to-gate spacing imposes constraints on layout, and can waste area. Therefore, it would be desirable to remove the need for this design rule.
In order to address this issue, as well as reflective notching of the polycide gate during patterning,.sup.1 a thick oxide has been deposited on top of the polycide prior to pattern and etch. Although this buys more margin, it still does not resolve the problem. FNT .sup.1 Where the material underlying a photoresist layer is reflective and nonplanar, light may be reflected laterally from a more-illuminated location to a less-illuminated location, degrading the accuracy of patterning.
Another method uses a second poly as the "landing pads" for bitline contacts in the matrix. In this way, the wordlines are protected thereby allowing for zero contact-to-gate spacing. The drawback is that this method cannot be used in the periphery due to contact resistance problems which result in slower operation of the device.
Another method uses a silicon nitride deposition on top of the polycide prior to pattern and etch. (See Singer, "A New Technology for Oxide Contact and Via Etch", SEMICONDUCTOR INTERNATIONAL, August 1993, p.36, which is hereby incorporated by reference.) Through the use of highly selective nitride to oxide etch selectivities, this will reduce the possibility of the contact touching the top of the polycide; however, it can still make contact along the sidewall due to the removal of the oxide spacer. Additionally, this does not address the problem of overetching the field oxide during the spacer etch.
Still another proposed method uses an Al.sub.2 O.sub.3 etch-stop layer to achieve a zero-margin contact process. (See Fukase et al., "A Margin-Free Contact Process Using an Al.sub.2 O.sub.3 Etch-Stop Layer for High-Density Devices," 1992 IEDM PROCEEDINGS 33.3, which is hereby incorporated by reference.)
The disclosed inventions describe new methods of forming self-aligned aligned contact utilizing Si.sub.3 N.sub.4 (or other dielectric film with lower reflectivity than WSi.sub.x film and good etch selectivity to oxide film; e.g. oxynitride, metallic oxides, etc.). The Si.sub.3 N.sub.4 film can be used as follows:
1. serve as the anti-reflecting coating ("ARC") for gate patterning PA1 2. gate spacer for LDD (lightly doped drain) PA1 3. contact etch stop layer.
The disclosed innovative techniques are used to simultaneously form contacts to multiple levels of poly/polycide, as well as to active areas (both for multiple poly interconnecting and metal interconnects), with zero contact-to-poly spacing. This is a significant advantage. Furthermore, this technique can be applied both in the memory array and in the circuit periphery without any degradation in circuit performance.
To avoid problems due to nitride sidewall spacers a polysilicon reoxidation step is performed, after the gate etch, to ensure that a pad oxide is present under the nitride spacers which are then formed on the sidewalls of the gate. This helps to avoid accumulated charge due to trapping of hot carriers.
According to a disclosed class of innovative embodiments, there is provided: An integrated circuit fabrication method, comprising the steps of: providing a substrate which includes exposed surface portions of substantially monolithic semiconductor material separated by regions of field dielectric; forming a gate dielectric on said surface portions; forming a first thin-film conductor layer over said gate dielectric and said field dielectric; forming a first thin-film dielectric layer over said first conductor layer; selectively removing said first dielectric layer from locations where contact to said first conductor layer will be formed over said field dielectric; anisotropically etching said first conductor layer, together with said first dielectric layer thereover, to leave said conductor layer in a pattern which provides transistor gates in desired locations, and also exposes locations where contact will be formed to said surface portions of said semiconductor material; performing oxidation, to grow an oxide layer on exposed portions of said conductor layer and said surface portions; depositing a second layer of dielectric material overall, and anisotropically etching said second layer to leave sidewall spacers adjacent to edges of said conductor layer; depositing an interlevel dielectric overall, said interlevel dielectric having a composition which is significantly different from that of said first and second dielectric layers, and etching to expose contact locations, using an etch chemistry which etches said interlevel dielectric selectively with respect to said first and second dielectrics; and depositing a second thin-film conductor layer to make contact to exposed portions of said first conductor layer and said semiconductor material.
According to another disclosed class of innovative embodiments, there is provided: An integrated circuit fabrication method, comprising the steps of: providing a substrate which includes exposed surface portions of substantially monolithic semiconductor material separated by regions of field dielectric; forming a gate dielectric on said surface portions; forming a first thin-film conductor layer over said gate dielectric and said field dielectric; forming a first thin-film dielectric layer over said first conductor layer; selectively removing said first dielectric layer from locations where contact to said first conductor layer will be formed over said field dielectric; anisotropically etching said first conductor layer, together with said first dielectric layer thereover, to leave said conductor layer in a pattern which provides transistor gates in desired locations, and also exposes locations where contact will be formed to said surface portions of said semiconductor material; performing oxidation, to grow an oxide layer on exposed portions of said conductor layer and said surface portions; depositing a second layer of dielectric material overall, and anisotropically etching said second layer to leave sidewall spacers adjacent to edges of said conductor layer; depositing overall an interlevel dielectric predominantly comprising silicon oxides, and etching to expose contact locations, using an etch chemistry which etches said interlevel dielectric selectively with respect to said first and second dielectrics; and depositing a second thin-film conductor layer to make contact to exposed portions of said first conductor layer and said semiconductor material, and patterning said second conductor layer to implement a desired pattern of electrical interconnection.
According to another disclosed class of innovative embodiments, there is provided: An integrated circuit fabrication method, comprising the steps of: providing a substrate which includes exposed surface portions of substantially monolithic semiconductor material separated by regions of field dielectric; forming a gate dielectric on said surface portions; forming a first thin-film conductor layer over said gate dielectric and said field dielectric; forming a first thin-film dielectric layer over said first conductor layer; selectively removing said first dielectric layer from locations where contact to said first conductor layer will be formed over said field dielectric; anisotropically etching said first conductor layer, together with said first dielectric layer thereover, to leave said conductor layer in a pattern which provides transistor gates in desired locations, and also exposes locations where contact will be formed to said surface portions of said semiconductor material; performing oxidation, to grow an oxide layer on exposed portions of said conductor layer and said surface portions; depositing a second layer of dielectric material overall, and anisotropically etching said second layer to leave sidewall spacers adjacent to edges of said conductor layer; depositing overall an interlevel dielectric predominantly comprising silicon oxides, and etching to expose contact locations, using an etch chemistry which etches said interlevel dielectric selectively with respect to said first and second dielectrics; and depositing a second thin-film conductor layer to make contact to exposed portions of said first conductor layer and said semiconductor material, and patterning said second conductor layer to implement a desired pattern of electrical interconnection.
According to another disclosed class of innovative embodiments, there is provided: An integrated circuit, comprising: a body of monocrystalline semiconductor material, having active areas therein separated by field dielectric regions of a first dielectric material; a first patterned thin-film conductor layer, capacitively coupled to said semiconductor material in multiple transistor channel locations in said active areas, and also running across portions of said field dielectric regions; an interlevel dielectric material overlying said first thin-film conductor layer, and having contact holes therein; a second patterned thin-film conductor layer overlying said interlevel dielectric material, and extending down through said contact holes to make contact to said first conductor layer in selected locations, and also to said active areas in selected locations; wherein a layer of a second dielectric material, which is different from said first dielectric material and from said interlevel dielectric material, overlies said first thin-film conductor, beneath said interlevel dielectric; and wherein sidewall spacers of said second dielectric material abut sidewalls of said first conductor layer at said selected locations.